Carbon dioxide supplied to food and beverage customers normally meets a purity specification known as Enhanced Ingredient Grade (EIG). Carbon dioxide of this purity is sufficient for use in food and beverages, and most plants can produce it.
Some applications require carbon dioxide of higher purity than EIG. For example, supercritical fluid extraction and supercritical fluid chromatography require small amounts of higher purity carbon dioxide. Examples of the types of higher purity carbon dioxide include Supercritical Fluid Extraction (SFE) and Supercritical Fluid Chromatography (SFC) grades. These grades of carbon dioxide are generally supplied in cylinders.
Additional applications requiring high purity and ultra-high purity (UHP) carbon dioxide have recently been developed. These include pharmaceutical processing, semiconductor processing (e.g., photoresist removal and wafer cleaning), micro-electromechanical systems (MEMS) drying, and metal target cleaning.
Previous methods for purifying carbon dioxide include distillation. Single-stage distillation is a commonly employed technique, in which carbon dioxide vapor is taken from a bulk tank or a cylinder and passed through a condensing unit, where it partially condenses. Heavy contaminants concentrate in the resulting liquid phase. The resulting purified vapor phase is sent to the application. By drawing from the vapor space, the concentration of contaminants is reduced by about one order of magnitude over that in delivered liquid. The efficacy of this approach depends on the condensation pressure. As this pressure increases, the concentration of heavy components in the vapor phase increases. Thus, a better separation is achieved at lower pressures. Unfortunately, carbon dioxide does not exist as a liquid at low pressures, rendering single-stage distillation ineffective for those pressures. At those pressures for which single-stage distillation is effective, the degree of contaminant removal is unacceptable for applications requiring high purity and UHP carbon dioxide. In addition, as the vapor is removed from the cylinder the pressure drops. If the pressure drops quickly, liquid boiling will cause significant turbulence. This turbulence can project fine droplets into the vapor space. These droplets, which contain elevated levels of contamination, are withdrawn along with the vapor.
Another approach to removing contaminants from carbon dioxide is adsorption. Carbon dioxide is passed through a bed containing adsorptive material, such as activated carbon. The contaminants are adsorbed onto the material, and carbon dioxide flows out of the bed with fewer contaminants.
U.S. Pat. No. 5,470,154 discloses the use of two adsorbent beds to remove contaminants and produce 99.999% carbon dioxide. One bed contains molecular sieves or alumina and the other contains activated carbon. This purification is to be done at close to atmospheric pressure, and is likely to be ineffective for removing certain types of contaminants.
R. Zito, “CO2 Snow Cleaning of Optics”, Proceedings of SPIE, Vol. 4096 (2000) discloses a means of using an adsorption bed to solve the problem of providing UHP carbon dioxide for precision cleaning of optics using carbon dioxide snow.
U.S. Pat. No. 6,099,619 discloses a point-of-use purifier to produce high-purity carbon dioxide vapor by passing carbon dioxide through an adsorbent bed or beds for purification. The adsorbent bed is sized depending on the amount and quality of carbon dioxide needed.
Adsorbent beds have several disadvantages when used to clean carbon dioxide. First, adsorbent beds are not effective for removing certain types of contaminants from carbon dioxide because most adsorbents have a strong affinity for both the carbon dioxide and the contaminants. This leads to competition for adsorption sites, and decreased adsorbent ability. Secondly, adsorbent beds are not effective for removing certain sizes of contaminants from carbon dioxide. Contamination can include a broad range of molecules in terms of both size and composition, and it is difficult to design an adsorbent train that will capture all such molecules so that the carbon dioxide reaches the required purity.
In liquid absorptive purification, contaminant-containing carbon dioxide vapor is contacted with clean carbon dioxide liquid. The contaminant in the carbon dioxide vapor generally migrates to the liquid carbon dioxide to some extent, purifying the carbon dioxide vapor. The efficiency of purification depends on the ability of the system to effectively contact the solid particles and aerosol droplets in the gas with the liquid carbon dioxide. Also, the relative affinity of the contaminants for the vapor and liquid carbon dioxide will affect the ability of this purification method. Hence, this approach is generally ineffective for removing those contaminants that have little or no relative affinity for carbon dioxide in the liquid phase.
A particulate filter may be used to remove contaminants from carbon dioxide. U.S. Pat. No. 4,972,677 discloses vaporization of liquid carbon dioxide prior to passing it through a particulate filter for purification. The resulting purified vapor is then re-condensed. Downstream of the filter, the carbon dioxide flows through equipment having ultra smooth inner surfaces cleaned by electro or chemical polishing. Generally, particulate filters can remove solid contaminants but are inefficient for the removal of contaminants in a liquid or vapor phase. Even if a liquid aerosol droplet is larger than the pore size of the filter, the droplet can push through the pores of the filter and re-entrain in the gas downstream of the filter.
U.S. Pat. No. 4,759,782 discloses the use of a coalescing filter to remove liquid contaminant particles, such as water and oil, from compressed gas streams. A coalescing filter is different than a particulate filter. Liquid droplets collect on the coalescent filter element and merge together to form larger droplets. Eventually, the merged droplets grow large enough to fall to the bottom of the coalescing filter housing where they can then be removed. Coalescing filters are effective for removal of solids and liquid particles, but vapor molecules pass through them. The stream leaving the coalescer has reduced solid or liquid particles but tends to be saturated with vapor phase contaminant. In addition, when the amount of solids is high, coalescing filters must be protected with a particulate filter to prevent them from becoming clogged.
U.S. Pat. No. 6,327,872 discloses a delivery system aimed at serving applications that require high purity carbon dioxide. A filter (coalescing and/or adsorbent filters) is used to increase the purity of carbon dioxide vapor. The vapor is then condensed in a condenser and directed to two tanks at a relatively low pressure. Heating the tanks increases the pressure. By using two tanks, a constant flow of liquid can be delivered without the use of compressors or pumps. This method is generally unable to remove contaminants that are in a vapor phase and/or are not removed by the adsorbent filter.
U.S. Pat. No. 5,976,221 discloses the use of coalescing filters followed by an adsorbent bed containing a macroporous polymeric material to remove oil from compressed air. The coalescers are used to reduce the liquid oil content in air coming from the compressor from 5 to 10 ppm by weight to less than 1 ppm. The macroporous adsorbent is used to reduce the amount of liquid oil and oil vapor from less than 1 ppm to less than 10 ppb.
Thermal catalytic oxidation is a process by which hydrocarbons are removed from a gas by reaction with an oxidant, such as oxygen, to form carbon dioxide and water. Kohl and Nielsen, “Gas Purification”, 5th Ed., Gulf Publishing Company, Houston (1997) disclose a generic thermal catalytic oxidation system for use in removing volatile organic compounds (VOCs) from an air stream. The thermal catalytic oxidation system has three unit operations: a heat exchanger, a burner, and a catalyst bed. The air that is to be purified first passes through one side of the heat exchanger where it is heated by indirect contact with hot gas leaving the catalyst bed. The preheated air then flows to the catalyst bed where its temperature is raised further by mixing it with hot combustion gases from the burner. The hot air flows across the catalyst where the VOCs react with oxygen to form carbon dioxide and water. Heat is released by this reaction, and the temperature of the air stream increases. The hot purified air exits the catalyst bed and flows into the heat exchanger where it is cooled by indirect contact with the incoming air. The catalyst includes a platinum group metal deposited on an alumina support. The support is either in the form of pellets that are arranged in a packed reactor bed or in the form of a monolithic structure whose passages are coated with the catalyst material. Older designs used the pellet catalysts exclusively, but more modern systems often employ monoliths. Because the destruction of low molecular weight hydrocarbon (e.g., methane, ethane) requires a large amount of energy, the system taught in Kohl and Nielsen must be operated at very high temperatures.
U.S. Pat. No. 5,612,011 describes a process for purifying an inert gas leaving a solid-state polycondensation reactor. The inert gas is mixed with an oxygen-containing gas and sent to a thermal catalytic oxidation system. The thermal catalytic oxidation system comprises a platinum or platinum-based catalytic bed operating at temperatures between 250 and 600° C. Purified gas leaving the bed is directed to a solid-state polycondensation reactor to eliminate the water formed during the oxidation process.
U.S. Pat. No. 5,738,835 describes a process for purifying a gas from the thermal solid-phase treatment of condensation polymers. This gas is mixed with an oxygen-containing gas and added to a thermal catalytic oxidation system. Oxygen-containing gas is added such that the carbon monoxide level in the stream leaving the thermal catalytic oxidation system is maintained between 30 and 100 volumes per million.
Both the '011 and '835 patents control the temperature of the thermal catalytic oxidation unit based on the carbon monoxide content in the effluent. However, this approach is not appropriate for controlling the level of some types of contaminants.
U.S. Pat. No. 5,914,091 describes a point-of-use thermal catalytic oxidation system for treatment of gaseous waste streams leaving a semiconductor manufacturing process units. The waste stream is pressurized using a fan or blower, then warmed to the thermal catalytic oxidation unit operating temperature by a heat exchanger and supplemental heater. It then enters the thermal catalytic oxidation unit. The resulting purified stream is cooled to near ambient temperature in the heat exchanger before it is vented. The heat exchanger and supplemental heater are arranged to effect autothermal catalytic oxidation of VOCs contained in the gaseous waste stream. By definition, VOCs include substances such as methane and ethane. As a result, the thermal catalytic oxidation unit must be operated at very high temperatures in order to ensure oxidation of VOCs.
WO 02/085528 A2 describes a process for treatment of carbon dioxide that is used in dense phase applications. Untreated carbon dioxide is sent to a coalescing filter to remove gross hydrocarbon impurities. It is then sent to a membrane filter to remove water. Finally, it is sent to a photo- or thermal-catalytic treatment unit to remove light hydrocarbons at sub-critical pressures. The resulting purified carbon dioxide is sent to a filter to remove any remaining impurities. WO 02/085528 A2 states that the purpose of its thermal catalytic oxidation unit is to remove all hydrocarbons and specifically states that it removes volatile hydrocarbons. Therefore, it must operate at very high temperatures. Also, a hydrocarbon analyzer is used to measure complete volatile hydrocarbon removal, which is not appropriate for the removal of some types of contaminants.
Therefore a need exists for methods and systems that reduce or minimize the above-referenced problems.